use os/user to determine home directory on Windows
consolidate homePath() logic in hkclient as HomePath()
display of SSL endpoint info, ability to add certs

Added ssl and ssl-cert-add commands.
add ssl-cert-rollback command
cleanup addon-destroy warning
add ssl-destroy command to remove an SSL endpoint
zsh completion for ssl commands

Fixes #143.
complete all files, not just .pem (thanks @dpiddy)
handle the nonexistent netrc case inside hkclient
use fatal errors in auth where appropriate
Only read body for PATCH/PUT/POST.
Add addon-plan-change command

Enable users to change (upgrade/downgrade) an addon's plan.